Onset-withdrawal dates of autumn persistent rains over western China and the associated autumn to winter evolution of the atmospheric circulation

Based on the NCEP/NCAR reanalysis pentad dataset (1961-2010), the CMAP pentad rainfall dataset (1979-2008), and the conventional pentad rainfall records (1961-2010) over the 553 stations in China, the relationship between the autumn to winter evolution of the atmospheric circulation and onset-withdrawal dates of the autumn persistent rains (APR) over western China (WC) is investigated. The results show that the seasonal variation of rainfall over the WC is characterized by double peaks, one in summer and the other in autumn. The trough between the peaks happens in early August (pentad 44), while the rainfall decreases to below its annual mean in mid-October (pentad 57). Therefore, the onset and withdrawal dates of APR are defined in mid-August (pentad 45) and mid-October (pentad 57), respectively.The onset of APR corresponds with the starting of the transition from the east Asian summer monsoon (EASM) to winter monsoon (EAWM), with the landmark circulation change being SW to SE wind shift at 850 hPa over south of the middle and lower reaches of the Yangtze River (SMLRYR). The positive to negative reversal of the meridional SLP gradient (SLP in the sea minus SLP over the land) in East Asia happens in mid-August (pentad 45), causing the 850 hPa SW wind over the SMLRYR replaced by the SE wind. Such a SE wind will meet over the WC with the SW wind from the Bay of Bengal (BOB), thus forming convergence there both in the wind direction and water vapor. As a result, the rainfall over WC will strengthen again after its summer peak and the APR is thereby established.The ending of APR is associated with the retreat of the tropical SW monsoon from the BOB and the complete establishment of EAWM, with the landmark circulation change being SW to NE wind shift at 850 hPa over the BOB. As the negative to positive reversal of the zonal SLP gradient (SLP over land minus SLP in the sea) in East Asia occurs progressively from the north to the south, the EAWM will advance southward, with the NE wind reaching the SMLRYR in early September (pentad 51) and the South China Sea in mid-October (pentad 57). The SW wind from the BOB disappears, the WC is fully controlled by the continental cold high, and the monsoon meridional circulation changes into the winter-type Hadley Circulation in East Asia, a situation marking the complete establishment of EAWM and the withdrawal of APR.Therefore, the onset-withdrawal of APR may be associated with the asynchronous summer to winter transition between the EASM and the south Asia summer monsoon (SASM). Due to the cooperative contribution of the EASM and SASM, the APR thus becomes the last rainy season of the Asian summer monsoon in mainland China.